1. Field of the Invention
The present invention relates to a motor cooling system used as a traction power source of a hybrid vehicle and the like, and to a technology for effectively cooling a neutral point.
2. Description of the Related Art
A hybrid vehicle mounts an engine (internal combustion engine) and a traction motor as a traction power source and appropriately uses both or one of these, thereby realizing a highly efficient driving. For example, using exclusively the engine in cruising on a highway and the like, and both of the engine and the traction motor in an acceleration drive and a hill climb drive, a good fuel consumption and a high driving performance are realized. As a traction motor of the hybrid vehicle, a three-phase alternating current motor of a star-connection having a neutral point is generally used because of its high efficiency and control easiness. In this kind of a three-phase alternating current motor, because the neutral point bundles a wiring of each phase and is formed with being caulked by a harness, an electric resistance at the neutral point becomes larger, compared to other portions, and a density of strands becomes higher. Therefore, if high-load driving such as an acceleration drive and a hill climb drive is performed in a hybrid vehicle, heat radiation is not smoothly performed in spite of a large amount of heat generated accompanying an increased load at the neutral point (that is, an increase of a consumption current) because of the high density of the strands, and a temperature rises until near an allowable limit in a comparatively short period of time. In this case a controller of the hybrid vehicle is designed to drive by an engine only, for example, regulating a power output of the traction motor, in order to suppress the temperature rise of the neutral point.
In the three-phase alternating current motor a structure is desirable that can suppress the temperature rise of the neutral point in order to prevent the hybrid vehicle from resulting in regulating a power output thereof, shortening its life, and the like. As a technology for suppressing the temperature the neutral point is proposed one that: a plurality of neutral points are provided in a three-phase alternating current motor; the neutral points are connected by a crossover having a diameter not less than a wire diameter of a winding wire of each phase; and thereby a heat concentration on the neutral points is suppressed (for example, see paragraph 0007 and FIG. 2 of Japanese Patent Laid-Open Publication Hei. 10-80085). On the other hand, in a three-phase alternating current motor for a hybrid vehicle is proposed a technology that: a gap is provided between an armature core and a cover; a hydraulic oil of a clutch, which configures a power force output device together with the three-phase alternating current motor, is introduced into the gap; the hydraulic oil is supplied to coil end of a stator, and thereby cooling the motor is performed (for example, see paragraph 0082 and FIG. 9 of Japanese Patent Laid-Open Publication Hei. 11-206063).
However, in the three-phase alternating current motor of the Japanese Patent Laid-Open Publication Hei. 10-80085, because a manufacturing and the like of the winding wires and the crossover are difficult and a number of components also becomes many, manufacturing cost thereof increases and it is also difficult to ensure a reliability thereof. In addition, even if adopting such the structure, it is actually difficult to obtain a sufficient heat resistance and radiation. On the other hand, in the three-phase alternating current motor of the Japanese Patent Laid-Open Publication Hei. 11-206063, when hydraulic oil introduced is low in temperature, the neutral points can be effectively cooled; when hydraulic oil introduced becomes high in temperature, on the contrary the temperature thereof is made to rise in some case. For example, although when a hybrid vehicle cruises on a highway and the like, a load of the traction motor is comparatively small and a heat generation amount of the neutral points is a few, a power transmission mechanism such as a clutch and a transmission become very high in the temperature of the hydraulic oil by a friction heat increase, which accompanies a rise of a driving speed. Therefore, if the hydraulic oil of the high temperature is supplied to the traction motor, the temperature of the neutral points results in rising on the contrary.
Consequently, a motor cooling system is strongly requested that effectively cools a neutral point of a traction motor mounted on a hybrid vehicle and the like.